Conventionally, in a display element that displays numerals and letters or that performs surface emission or the like by using a point light source such as a light-emitting diode or a linear light source such as a fluorescent lamp or a cathode tube, a light diffuser film is used which scatters transmitting light as if the above-described point or linear light source is a surface light source.
In addition, improvement of uneven luminance and enhancement of luminance are also desired in terms of display performance in the above-described display element and also in a flat panel display (FPD).
A general backlight unit of a liquid crystal display is provided with a light guide plate that has light scattering dots printed on the back, a light source (cold cathode tube, etc.) disposed on one or both sides of said light guide plate, a lens film (prism sheet) layered on said light guide plate, and a light diffuser sheet disposed on top or on both top and bottom of said lens film as needed. The light diffuser sheet incorporated in such a backlight unit plays the roles of uniformly diffusing the light from the light guide plate to prevent the dots from being visible on the display screen and suppressing optical loss for uniformly emitting the diffused light to the liquid crystal panel surface.
Several technical problems, however, have been confirmed in using a light diffuser sheet in a liquid crystal panel. Major problems include a phenomenon called glare caused by occurrence of a luminance distribution resulting from the interference between the color filter and the uneven surface of the optical film of the image display device, and a moire phenomenon caused by the interference between the uneven luminance and the color filter. Thus, it is desirable to suppress occurrence of these phenomena.
As a measure for reducing the glare, an anti-glare film described in Prior art document 1 is known, with which glare caused by an uneven profile having a period of 10 to 50 μm is reduced. The haze value, however, is about 1%, which is inadequate in terms of diffusion performance for suppressing the moire phenomenon.
Meanwhile, Prior art document 2 describes a film having a diffusion property sufficient to eliminate the moire phenomenon, but the profile of the film has a small number of valley parts per unit area and thus uniformity of the brightness is insufficient to avoid occurrence of glare.
Hence, there has been no film that overcame glare and moire at the same time by conventional technique.